1. Field of the Invention
The present invention relates to a method and apparatus for withdrawing a measured fluid sample from a container and, more particularly, for use in an automated blood/plasma sampling system, for example as disclosed in U.S. patent application Ser. No. 07/443,951, filed Dec. 1, 1989, by William C. Hulette, et al. assigned to the same assignee as the present application the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
Fluid samples, such as blood or plasma, are stored in containers, such as test tubes, which are vacuum sealed by way of a rubber septum that must be pierced in order to withdraw a measured amount of the sample for testing purposes.
Known needles or piercing and sampling probes withdraw samples from these vacuum sealed containers, e.g. Vacutainers (TM), by means of a two part device. These sampling devices include an outer piercing sheath having a beveled edge for piercing the septum and, an inner, more narrow sampling probe which may be lowered through the outer sheath into the vacuum sealed container to withdraw a measured amount of fluid for testing. In this known mechanism, the tip of the inner, more narrow sampling probe comprises a flat edge, as it is not required to pierce the septum. This flat edge provides greater accuracy in the withdrawing and delivery of the fluid sample.
However, there are several disadvantages with this type of coaxial piercing and sampling probe arrangement. First, it involves a two step process, piercing the septum with the outer, larger diameter sheath and lowering the inner, more narrow sampling probe through the sheath into the vacuum sealed container to withdraw the test sample. This process is somewhat cumbersome as it requires two separate drive mechanisms, one for the outer sheath and another for the sampling probe itself. This complexity adds to the overall expense of the device.
Another known device involves a piercing and sampling probe having a single needle with a beveled edge which is used to pierce the septum of the vacuum sealed container and through which a measured fluid sample may be withdrawn. One problem with this device is that for small sample volumes, the angle of the bevel needed to pierce the septum is too steep so that a lot of dead volume remains in the sample container. That is, when the needle abuts the bottom of the sample container, the height of the elliptical opening at the needle tip causes the needle to begin drawing in air while a relatively large residual volume of sample still remains in the container.